1. Field of the Invention
The invention relates to the field of fabrication processes and structures for quantum dots, and in particular, for semiconductor quantum dots and the process for making the same using strains created in the epitaxial growth layer as a result of differences in crystalline lattice spacing.
2. Description of the Prior Art
The fabrication of very small semiconductor structures to exploit the quantum mechanical behavior of carriers within semiconductor material is well established. However, the fabrication of practical structures for quantum confinement in two dimensions (quantum wire) and in three dimensions (quantum dots) has generally been unsatisfactory. In an attempt to attain the highest levels of confinement, as in a quantum dot, several methods have been used, including growth on vicihal surfaces. See O. Brantit et al., Phys. Rev. B 44, 8043 (1991) for the use of cluster formation from solution precursors; see M. L. Steigerwald et al., Ann. Rev. Mater. Sci. 19, 471 (1989), and M. A. Olshavsky et al., J. Amer. Chem. Soc. 112, 9438 (1990), for gas-phase nucleation; see A. Saunders et al., Appl. Phys. Lett. 60, 950 (1992), see G. D. Stucky et al., Science 247, 669 (1990), for loading of semiconductors and mesoporous cages; and for several other ex-situ processing techniques, see Optics of Excitons in Confined Systems, A. Quatropani Editor Institute of Physics, London, (1991). Most of these prior techniques suffer from poor interface quality and size nonuniformity giving rise to large surface recombination velocities.
Highly strained heteroepitaxial growth generally proceeds by the formation of islands after initial layer-by-layer growth which is referred to as StranskiKrastanow growth. Recent studies of indium gallium arsenide, (InGaAs) on gallium arsenide (GaAs) by scanning tunneling microscopy and transmission electron microscopy and of germanium/silicon (Ge/Si) by transmission electron microscopy have demonstrated that at the initial stages of formation, these islands are dislocation-free and coherently strained to the substrate, See S. Guha et al., Appl. Phys. Lett. 57, 2110 (1990); for transmission electron microscopy see D. J. Eaglesham et al., Phys. Rev. Lett. 64, 1943 (1990); and for transmission electron microscopy in Ge/Sisee M. Krishnamurthy et al., J. Appl. Phys. 69, 6461 (1991) and B. G. Orr et al., Euro Phys. Lett. 19, 33 (1992).
Though a clear mechanism for this strain relief has yet to emerge, it is thought to occur by elastic relaxation of the nearest substrate layer or by kinetics of strain induced surface roughening, C. W. Syder et al., Phys. Rev. Lett. 66,3032 (1991). It is clear that at the initial stages of growth, introduction of dislocations arising out of misfit relief can be avoided. At the later stages of growth, dislocations may form when the islands coalesce, leading to the degradation of the layer. Consequently, most of the studies have concentrated on suppressing island formation, such as shown by M. Kopel et al., Phys. Rev. Lett. 63, 632 (1989), or on using growth kinetics to extend the onset of islanding. Studies of the microstructure evolution of coherent islands have been reported for germanium on silicon by M. Krishnamurthy et al. supra and M. Tabuchi et al., Science and Technology Mesoscopic Structures, S. Namba et al. editor (Springer, Tokyo 1992) at page 379. It was shown that under certain growth conditions, sharp island-size distributions could be obtained, M. Krishnamurthy et al. supra. A study of the InAs three dimensional islands on GaAs surfaces was performed and the optical properties were reported, although island-size distributions or defects associated with them were not considered.
What is needed then is a methodology for fabricating quantum dot structures without any of the additional processing steps which the prior art studies would dictate is required, and for fabricating quantum dot structures with a uniformity of defect-free dots with sharply peaked, size distributions.